Progress in the Chemistry and Molecular Biology of Flavins and Flavocoenzymes

Abstract

An attempt is made to create a basis for the understanding offlavin-(Vitamin B2-) dependent oxidations in terms of molecular structure. The main points of this survey can be summarized as follows: a) Theflavins must be differentiated according to their redox states (quinone state, radical state, hydroquinone state), since configuration, energy content of the tautomeric forms, metalaffinity etc., alter drastically during reduction. -6 ) The methylgroup on C-8 of the flavoquinone is reactive. -cj The electron transfer properties of the flavin system frequently depend on the coupling of secondary redox systems to the apoprotein. This interaction thermodynamically stabilizes the half-reduced state of the flavin. -d) Numerous novel ffavin types, among them stabilized free radicals, have become availabIe through syntheses at the dihydro stage. -ej Reaction of the flavin radical with 0 2 is slow as long as no disproportionation occurs, i. e. the fast flavin autoxidation proceeds via the dihydro state.f l The flavin nucleus gains affinity for heavy metal ions only by the acceptance of one, and only one, electron. -g) Redox activity and contact with the ffavin coenzyme have been confirmed for Fe and Mo, which occur in flavoproteins. The properties of the metal coordination sphere are determined primarily by sulfur-containing groups of the apoprotein.

The electrochemical behavior of flavin adenine dinucleotide in neutral solutions

“…This is somewhat larger than the expected value for a reversible two-electron transfer reaction, 5712 mV, where z is the number of electrons transferred in the process (2). At sweep rates greater than about 20 mV/s, the peak separation begins to increase, as is predicted for the onset of kinetic irreversibility.…”

“…These proteins utilize the cofactor flavin adenine dinucleotide (FAD) to dehydrogenate (oxidize) pyridine nucleotides and hydrogenate (reduce) the cytochromes (1,2). This valuable cofactor, which is derived from riboflavin (RF) (Vitamin B2), is unique in that, depending on the conditions, it can undergo either two separate one-electron transfer reactions or one essentially simultaneous two-electron transfer reaction.…”

“…Attached to this ring system is another group, depicted as R in Fig. 1 , which aids in apoprotein-cofactor binding and specificity (1,2). The isoalloxazine ring system can exist in an oxidized form (quinone), the radical semiquinone form, after the acceptance of the first electron, and the reduced hydroquinone form, after a second electron transfer.…”

“…It is also known that, in solution, the oxidized flavins are expected to have a planar structure, while the reduced flavin resembles a butterfly in conformation (2), with atoms 1 to 5 and 10, and 5 to 9 and 10 ( Fig. 1) constituting two planes or wings intersecting the N(5)-N(10) axis, with a dihedral angle of 21" (4).…”

Electrochemical Evidences for the Enhancement of Heterogeneous Electron Transfer Rates of Riboflavin in the Presence of Copper

“…Riboflavin can also be described as a biological chelating ligand due to the existence of nitrogen and oxygen atoms on its structure that can act as coordinating sites for metal ion chelation. Previous studies suggest that the redox process of the flavins is thermodynamically reversible and involves formation of quinone, semiquinone and hydroquinone intermediates [2][3][4] . The electrochemical characterization of riboflavin has been examined by Zhang, H.et.al 5 .…”